1. Field of the Invention
This invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly to the system of the engine in which a blended fuel composed of, for example, gasoline and alcohol is used.
2. Description of the Prior Art
Recent years have seen increasing use of alcohol and the like as alternative fuels to replace traditional engine fuels such as gasoline. While such alternative fuels are sometimes used as they are without blending, they are also often used as mixed with a conventional fuel. One example is the hybrid fuel known as M85, which consists of 85% methanol (methyl alcohol) and the remainder of gasoline.
The stoichiometric air-fuel ratios of hybrid fuels generally differ from those of conventional fuels. In particular, the stoichiometric air-fuel ratio of methanol is about 6.5:1, considerably different than that of about 15:1 for gasoline. Moreover, alternative fuels obtained from different sources or at different times are frequently found to differ in blending ratio. It is thus highly possible that the blending ratio of an alternate fuel contained in the fuel tank of an engine will change regularly. Since any change in the blending ratio means a change in the stoichiometric air-fuel ratio, it is necessary to adjust the amount of fuel supplied to the engine accordingly.
With this in mind, Japanese Laid-Open Patent Publication No. Hei 1-244133 proposes an improvement in a known system in which the actual air-fuel ratio during engine operation is detected from, for example, the oxygen content of the engine exhaust gas and the detected air-fuel ratio is used as the basis for feedback control of the fuel supply, the improvement being that amount of fuel supply is optimized by using an alcohol sensor to detect the alcohol concentration of the fuel and adjusting the amount of fuel supply on the basis of the detected alcohol concentration.
In such a system, since the alcohol sensor becomes an indispensable element insofar as a hybrid fuel containing alcohol is used, it becomes impossible to optimize the amount of fuel supply to the engine when the alcohol sensor fails (malfunctions or breaks down), because in such cases the alcohol detector will either output no alcohol concentration detection value at all or output one that deviates from the actual alcohol concentration of the hybrid fuel. The aforesaid feedback control thus becomes impossible. This is a particular problem during engine startup and acceleration because determination of the amount of fuel supply by open loop control under such circumstances is apt to make continued engine operation impossible.